The invention relates generally to ultrasound image processing. More specifically, embodiments of the invention relate to systems and methods for focusing received ultrasound beams.
In ultrasound imaging, a transducer is used to transmit ultrasound beams into a medium such as the human body to be examined. The transmitted beams are reflected from various discontinuities in a region of the human body. Received ultrasound echoes are converted into electrical signals which undergo a number of processing steps and are eventually transformed into an image which can be displayed or printed for examination.
Ultrasound transducers are arrays of small rectangular piezoelectric elements. A subset of the elements in an array is used to transmit or receive an ultrasound beam and is called a transmit or receive aperture, respectively. The signal received by each element in the aperture is amplified and filtered, and input to a channel of a multi-channel receive beamformer.
The beamformer applies different delays and apodization weights to the signals from each element in an aperture in order to form beams focused along linear directions. To speed-up the image formation process, it is desirable to concurrently form from the same set of received signals, multiple beams focused along different directions (parallel beams). A straightforward implementation requires different channels to be used for each aperture element and each parallel beam, making the beamformer cost, size and power consumption increase in proportion to the product of the number of elements in a transducer array and the number of parallel beams. Because it is desirable to increase both the number of elements in the aperture and the number of parallel beams, the complexity of the beamformer becomes a limiting factor in ultrasound imaging systems.
To reduce the cost, size and power consumption of beamformers, it is known to share the hardware of a channel, or parts of a channel to form two or more parallel beams. However, this still requires one set of channel hardware components for each transducer element in an aperture
To reduce beamformer complexity, synthetic aperture techniques are used. Synthetic aperture uses multiple transmit events for each receive aperture, and after each transmit, the signal is received on a different subset of elements of the receive aperture. The beams are reconstructed from the signals received during the multiple transmit/receive events. This allows a reduction in the number of receive channels, but increases the complexity of ancillary circuits. This method also suffers from motion artifacts whose correction further complicates the beamformer circuit.
By summing signals from symmetrical elements with respect to the center of the aperture and using one channel for each sum of signals from two elements, the number of channels can be reduced by a factor of two. However, this approach only works when the beam is not steered and is not well suited for parallel beams.